Degradates: A Key to Assessing Herbicide Impacts in Streams

Location of study areas, stream sites, and associated contributing drainage basins sampled in 1989-1990 for the reconnaissance for herbicide degradates in Midwestern streams. Figure 1-A from Scribner and others, 1999.

In 1998 the U.S. Geological Survey conducted the first large-scale reconnaissance for herbicide degradates in streams. Degradates are the product of the environmental transformations of the parent pesticide, and they can have similar properties to the parent pesticide. Over 70 Midwestern streams were sampled as part of the reconnaissance. Results from the reconnaissance show that the concentrations of herbicide degradates were relatively high when compared to their parent compounds and that herbicide degradates make up a significant share of the total herbicide load in steams during post-application runoff events. Specific results from the reconnaissance include:

The median concentrations of most of the herbicide degradates were greater than the median concentrations of their parent pesticide.

Maximum concentrations of the herbicides acetochlor, atrazine, alachlor, cyanazine, and metolachlor all exceed the maximum concentration of their degradates.

The results suggest that studies in which only parent herbicide compounds are measured will not tell the complete story of the potential impacts of herbicide use on aquatic ecosystems. Including herbicide degradates is important because many of the herbicide degradates have similar toxicities as their parent compounds. Degradates are also important in ground-water studies.

These results are part of a much larger study that has sampled the same streams since 1989. The results from the long-term study suggest that since 1989 there has been some lessening of the high herbicide concentrations in Midwestern streams during runoff events after herbicides have been applied to agricultural fields. This good news is tempered by the discovery of relatively high concentrations of herbicide degradates in the 1998 samples. More information on the decline of herbicide concentrations in Midwestern streams is available.